Isotope variability of particulate organic matter at thePN section in the East China Sea

Y. WU1,*, J. ZHANG1,2, D.J. LI1, H. WEI3 and R.X. LU4

1State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai,200062, P.R. China; 2College of Chemistry and Chemical Engineering, Ocean University of Qingdao,Qingdao, 266003, P.R. China; 3Physical Oceanography Lab, Ocean University of Qingdao, Qingdao,266003, P.R. China; 4First Institute of Oceanography, SOA, Qingdao, 266071, P.R. China; *Author forcorrespondence (e-mail: wuying@sklec.ecnu.edu.cn; phone: +86-21-62232887; fax: +86-21-62546441)

Received 2 April 2002; accepted in revised form 26 February 2003

Key words: Carbon stable isotope, Nitrogen stable isotope, Particulate organic matter, East China Sea,PN section

Abstract. The source of particulate organic matter at the PN section in the East China Sea has beenevaluated using stable carbon and nitrogen isotopes. The results showed that the �13C and �15N com-positions varied from −19 to −31‰ and 0.7–9.5‰ respectively, and the isotope compositions were sta-tistically distinct, enabling, by use of a simple components mixing equations, assessment of the abilityof each tracer to estimate the terrestrial, Kuroshio Water, marine and remineralized sources’ contribu-tions. The dominance of terrestrial inputs of the Changjiang could be observed 250 km far from theriver mouth in the East China Sea. In the shelf water column, the remineralization of biogenic organicmatter becomes an important source except for the terrigenous and marine sources. The estimation ofsources recorded by �13C data was partly confirmed by equivalent �15N and C/N compositions thatreflected greater control by organic matter diagenesis and biological processing. However, the lightercontribution of �13C data of the Kuroshio samples also indicates the alteration of the isotope values bymicrobial or other processes. The net export flux of POC in the PN section is estimated to be 4.1kmol C/s and the annual export is 129 Gmol C/yr, which is account for 20% of the East China Sea.

Introduction

The continental margins of the ocean are characterized by very active biogeochem-ical processes – primary productivity generation, organic matter and nutrient sinksfor example (Wollast 1993; Hall et al. 1996). Scientific interest has been focusedrecently on the marginal seas with special emphasis on the exchange of carbon be-tween compartments, e.g., shelves and the open ocean, in order to better constrainthe continental margin carbon fluxes on a global scale (Walsh et al. 1988; SCOR1994; Chen and Wang 1999a; Liu et al. 1999). Many regional studies indicate thatactive cross-shelf transport and biogeochemical processes in the margins influencethe carbon cycle of the ocean as a whole (Tsunogai et al. 1999; Liu 2000; Liu et al.2000).

The East China Sea (ECS) is known for its broad continental shelf, rich naturalresources and tremendous river runoff from China (e.g., Changjiang (the Yangtze

© 2003 Kluwer Academic Publishers. Printed in the Netherlands.Biogeochemistry 65: 31–49, 2003.

River)). The riverine carbon fluxes comprise 12 Tg C yr−1 in organic form and 20Tg C yr−1 in dissolved inorganic form. The estimated organic carbon burial rate inthe ECS shelf sediments is no more than 10 Mt C yr−1 (Chen and Wang 1999a).On the east of ECS, the Kuroshio Current, a strong western boundary current,flowing along the Pacific Margin of northeastern Asia, borders the shelf slope ofthe East China Sea. When passing through the ECS, it brings forth great effects onthe ocean environment of the continental shelf area of the ECS. Such characteris-tics make the ECS as the key studying area for several decades (DeMaster et al.1985; Kim 1992; Ichikawa and Beadsley 1993; Liu et al. 1995; Chen et al. 1999b).

Although it has been well recognized that the Changjiang dilution and the Kuro-shio upwelling are two principal sources of materials of the ECS, few studies ad-dressed the processes of the transport of organic substance (e.g., carbon) and ex-change of materials, either dissolved or particulate, between various sources (suchas terrestrial and marine source) across the ECS shelf (Chen et al. 1995; Hung etal. 2000; Chen et al. 2001). For instance, the improved knowledge of the fates oforganic carbon is critical in understanding quantitatively the significance of terres-trial inputs and marine production in continental margins relative to global carboncycles (Liu et al. 1995; Chen and Wang 1999a; Chen et al. 1999b).

Hence, we examined isotopic compositions of carbon and nitrogen of the PNsection across the ECS shelf. The PN section starts off the Changjiang Estuary andends southeasterly at Ryukyu Islands (Figure 1). It crosses the Changjiang plumefront and shelf waters until Okinawa Through at the depth of 50–100 m of theKuroshio upwells along the margin until inner shelf. It is a typical area for thephysical, chemical and biological oceanographic interactions between Changjiangplume front, shelf water and Kuroshio. Studies were undertaken along the PN sec-tion with regards to volume, heat transport of the Kuroshio and interaction with theshelf water etc (Yuan et al. 1993; Sun and Kaneko 1993; Liu 2001).

In this study, the emphasis is given to the utilization of organic carbon and ni-trogen stable isotope ratios to identify provenance materials and organic mass flowand material flows in aquatic ecosystems and coastal ocean environment as previ-ously described in other world regions (Goering et al. 1990; Lucotte et al. 1991;Fry and Quinones 1994; Thornton and McManus 1994; Wu et al. 1999). Use ofstable isotopes relies on the implicit assumption that they are more conservativerelative to chemical composition, and hence distributions reflect mixing of materi-als from compositionally distinct end-member sources (Cifuentes et al. 1988). Theuncertainty of provenance discrimination is greatly reduced by simultaneous appli-cation of two or more isotope tracers (Cifuentes et al. 1988; Tan et al. 1991), thoughthe primary isotopic signatures can be obscured in hetertrophic event (e.g., catabo-lism) in early diagenesis (Benner et al. 1991; Montoya 1994).

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Materials and methods

The field observation was carried out along the PN section of R/V �Dong FangHong 2� cruise on October 20 – November 8, 2000 (Figure 1). A CTD-Rosette as-sembly with Niskin bottles was used to measure the profiles of temperature, salin-

Figure 1. Stations of survey reported in this study.

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ity and total suspended matter continuously along the depth in all stations. Water issampled for chlorophyll and POC/N, stable isotope ratios in most sample station atcertain depths (Figures 2d and 3). After collection, the water samples were filteredthrough Whatman 47 mm GF/F glass fiber filters with controlled pressure to avoidthe broken of cells. The filters were pre-heated at 450 °C to reduce backgroundcarbon level. After filtration, filters plus trapped particles were frozen and stored ina freezer. In the laboratory, the samples were dried in a freeze-dryer.

The contents of organic carbon and nitrogen were determined using a CHN el-emental analyzer (Carlo Erba 1108) after removing the carbonate fraction by leach-ing with 0.1 N HCl (Zhang et al. 1997). The detection limit is 1 × 10−5 with pre-cision of < 5–10%, estimated by repeated analysis. The carbon and nitrogen isotopiccompositions of POM were determined using attached to a DELTAplus/XL isotopicratio mass spectrometer (Finnigan MAT Com. USA) and a Carlo Erba 2500 ele-mental analyzer in combination. Duplicate determinations were made for each ofthe samples. The isotopic compositions of carbon (�13C) and nitrogen (�15N) areexpressed by:

�13C or �15N � �Rsample/Rstandard � 1.0��1000 (1)

where Rsample and Rstandard are the heavy (13C and 15N) to light (12C and 14N) iso-tope ratios of sample and standard, respectively. The standards for isotopic mea-sure are PDB for carbon and air for nitrogen (Fritz and Fontes 1980). The precisionof the analysis is ± 0.2‰ for �13C and ± 0.3‰ for �15N, respectively.

Results

Hydrographic properties

Vertical profiles of temperature, salinity, total suspended matter and chlorophyll awere obtained at the PN section of the ECS shelf. Temperature of surface waters(0–30 m) shows a vertically well mixed profile, increasing from 20 °C offChangjiang Estuary to 24 °C at the edge of the shelf (Figure 2a). At P2 station, thedepth of well-mixed profile of surface water temperature could reach around 80 m.Temperature decreased across the thermocline, which is normally at depth of 35 min this season. The near-bottom waters over the shelf at the depth (< 60 m) have anarrow range of temperature, i.e., 20–22 °C, typically at P3 and E7 stations. How-ever, the bottom water of Kuroshio has, however, much lower temperature, for ex-ample 15–17 °C at P2 station.

The salinity distribution along the PN section is shown in Figure 2b. In the dis-tance of 250 km off the Changjiang Estuary, there was a typical cross-section dis-tribution of salinity along the transect (increased from 26 to 33‰), which is quitedifferent from the distribution in summer. In the summer, the Changjiang effluentplume spread over the surface water of the continental shelf with stratified distri-bution (Liu 2001). In this study, it indicated the well mixing of fresh Changjiang

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Figure 2. Vertical sections of the distributions of temperature (a) and salinity (b) and total suspendedmatter (TSM) (c) and chlorophyll (d) for the PN section on the autumn cruise.

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Figure 2. Continued.

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Figure 3. Vertical sections of the distributions of POC (a) and �13C (b) and �15N (c) for the PN sectionon the autumn cruise.

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water with high saline shelf water off the Changjiang Estuary. The salinity distri-butions of surface water of the shelf showed less saline than the intruding KuroshioSurface Water (S = 34.4–34.6‰) at P2 station. The stratification structure of salin-ity distribution was observed in the outer shelf. There was a halocline at the depthof 40 m. At the bottom layer, there existed rather a uniform salinity (35–36‰), buta fairly strong vertical gradient (15–22 °C), which are characteristics of the Kuro-shio Subsurface Water (KSSW). The cool and saline bottom water in the outer shelfindicated intrusion of KSSW.

As expected, total suspended matter (TSM) distributions resemble the salinitydistributions off the Changjiang Estuary (Figure 2c). The concentration of TSMrapidly decreased along the transect of the shelf. The distribution of TSM of shelfwater was quite uniform in whole water column and just in a narrow range of 2–4mg/l. At the bottom of shelf edge, a relatively high TSM was observed, which couldbe attributed to resuspension due to sharp topography. In P2 station, TSM concen-trations in the whole water column were less than 2 mg/l.

The distribution of chlorophyll a showed strong stratified characteristics overthe whole shelf and Kuroshio water (Figure 2d). Out of the Changjiang estuary,chlorophyll a distributed as cross-section variation. Two chlorophyll a maximumexisted, one in Changjiang Estuary ( � 2 mg m−3) and the other on shelf surfacelayer ( � 4 mg m−3). In the upper 50 m shelf water (around euphotic zone), thechlorophyll a concentration varied in the narrow range of 1–3 mg m−3. Bottomchlorophyll a concentrations decreased to less than 1 mg m−3.

Obviously, several types of water mass can be identified along the PN sectionaccording to distributions of temperature, salinity, TSM and chlorophyll a. TheChangjiang plume front is characterized by low salinity as well as by high TSM,

Figure 3. Continued.

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which occupied mainly at Station E4 and P3. The shelf water is largely found inthe zone between Station P3 and E6. The water usually has a relative uniform tem-perature and salinity but low TSM. The Kuroshio surface water could be identifiedat P2 station surface layer because of high temperature and salinity. The intrusionof KSSW is also observed at the depth of 100–150 m of Station E6 and P2.

Organic carbon and nitrogen

Figure 3a is the POC profile in the PN section. The POC maximum occurred nearto the Changjiang Estuary with the high turbidity (50–60 µg/l). In the inner shelf,the concentration was homogenous at the surface layer. POC abundance generallydecreased seaward from the Changjiang Estuary to Kuroshio water (P2 station). Theminimum concentrations of POC (< 2 µg/l) were observed around P2 station. Arelatively high concentration of POC (15 µg/l) in the bottom water at the shelf edgeobviously resulted from sediment resuspension.

The distribution of particulate nitrogen (PN) resembled that of POC as shownabove. The correlation between POC and PN was very good (Figure 4). The cor-relation coefficient (R2) was 0.90. The slopes of the linear regression of PN againstPOC was 0.12, corresponding to C/N atomic ratios of 9.72.

POM �13C and �15N

Figure 3b–c presents vertical profiles of �13C and �15N of POM at the PN section.The value of �13C was the greatest in the mid shelf (−21 � −19‰) and decreasedat the surface layer in both eastwards and westwards. Out of the Changjiang Estu-ary, �13C values (−25 � −23‰) were observed homogenous in the whole watercolumn. A much lighter �13C values (−31 � −27‰) intruded to the shelf waterfrom the shelf slope at the mid layer. The downward decreasing trends were found

Figure 4. Particulate organic carbon content (µg/l) vs particulate nitrogen content (µg/l).

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in the �13C values at the mid and inner shelf except for the bottom water of Kuro-shio.

In contrast to �13C distribution, �15N profile is more complicated. Out of theChangjiang Estuary, minimum �15N value was observed and a cross-section vari-ation stopped at the distance of 100 km. The vertical distribution of �15N showedan increase with the depth at P3 and E7 station, but more fine structure was ob-served in E7 station with maximum values observed at the bottom layer. At E6 sta-tion, �15N values were homogenous at the upper layer and increased downward. Ahigher �15N observed at intermediate depth (50–75 m) for the whole water columnat Kuroshio Water.

Figure 5. Stable carbon isotope ratio of TSM vs Atomic carbon: nitrogen ratio (C/N).

Figure 6. Stable carbon isotope ratio vs stable nitrogen isotope ratio of TSM at the PN section.

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Discussions

Provenance of organic matter in the PN section

Particulate organic matterSeveral sources of organic carbon may contribute to the POM in the ECS shelf: (1)terrestrial organic carbon form the Changjiang Estuary; (2) alive or dead organicmatter of planktonic origin related to primary production; (3) the contribution fromthe Kuroshio water; (4) Resuspension of organic matter from the bottom sediments.The relative importance of (1) and (3) can be traced from the original data of POC(Figure 3a). The primary production in the East China Sea showed strong seasonalas well as spatial variation (Guo 1991). Its higher values were observed in summerand in the Changjiang Estuary (Wen 1995). In this study, higher Chl a values, whichwere in low concentrations (2.7–4.0 µg/l), were only observed in surface water overthe continental shelf. The average C/N ratio was just 9.72, which is quite differentfrom the redfield ratio of 6.63 (Redfield et al. 1963). All the above, the contributionof organic matter from in situ primary production is quite limited in autumn. Manystudies noticed high particulate fluxes and sedimentation rates observed on the shelfedge (Chung and Chang 1994; Chung and Hung 2000; Gao et al. 2000). The re-suspension mechanism is still not clearly identified but the significant flux from thebottom boundary layer could not be overlooked (Hung et al. 2000).

The C/N ratio is often indicative as the predominant source of organic matter ina system. Phytoplankton C/N ranges from 6–9 (Holligan et al. 1984). Bacteri-oplankton are nitrogen-rich and have C/N from 2.6–4.3 (Lee and Fuhrman 1987).The terrestrial organic matter can have significantly higher (C/N) (> 12; Hedges &Mann, 1979). The C/N ratios of POM along the PN section varied from 4.2 to 29.2(Figure 5), which were similar to the previous study in the ECS shelf (Liu et al.1998). Higher C/N values were detected from the bottom samples and samples col-lected at the outer shelf. Figure 5 indicates the relationship between stable carbonisotope ratios and C/N ratios. Normally, high C/N ratio can be indicative as terres-trial origin with much light �13C value. However, some POM of bottom water havehigh C/N ratios with heavy �13Cvalues and the POM of subsurface sample of P2station have low C/N ratios with much light �13C values. The decomposition pro-cesses (e.g., autolysis, leaching and microbial mineralization) should be the expla-nation of such variations (Roman 1980; Meyers et al. 1984). In general, the C/Nratio of POM might be expected to decrease or increase during decomposition withvarious sources. Such variation could make the C/N ratio superimposed upon theestimation of different sources of organic matter (Rice and Tenore 1981; Thorntonand McManus 1994).

Carbon isotope compositionsThe range of carbon isotopes ratios of organic matter is broad in aquatic ecosys-tems, ranging from −30 � −26‰ in runoff from terrestrial carbon sources to −22� −18‰ for carbon from phytoplankton production (Cifuentes et al. 1996). In theChangjiang Estuary, organic matter comes from river runoff, in situ production and

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sewage. The �13C values were more positive (−22 � −24‰) than expected forterrestrial organic matter (−27‰) because of the modification of sewage and in situproduction (Tan et al. 1991). In the shelf water, the variation of �13C values wasquite narrow from −19.8 to −23.7‰, which indicated the predominance of in situproduction. From the distribution of �13C values of POM of surface waters exceptP2 station, an increasement of �13C values was observed from the Changjiang Es-tuary to the shelf region. Theoretical considerations and laboratory studies suggestminimal changes in the �13C composition of organic detritus following extensiveand prolonged decomposition (Gearing et al. 1984). But the seaward 13C enrich-ment of POM observed here appears unlikely to reflect selective bacterially medi-ated removal of 12C from terrigenous carbon for the reason of the consistence ofthe known water circulation.

However, the more 13C – depleted values was observed in the subsurface waterof P2 station, which locates on the shelf slope, where the Kuroshio Subsurface Wa-ter intrudes into the shelf water. Such more negative values (indicating 13C deple-tion) were also observed in polar seas and have been attributed to an increase in thedissolved CO2 pool due to lower temperatures, which results in a greater isotopicfractionation during carbon assimilation (Rau et al. 1992). More depleted bacterial�13C values were detected from Gulf of Mexico, which suggests that carbon fromsources other than phytoplankton production or terrestrial organic matter supportproduction of the bacterial assemblage. Possible sources include light hydrocarbonsfrom seep areas (e.g., methane, ethane, propane and butane) and the chemoau-totrophic processes of methane oxidation and nitrification (Kelley and Coffin 1998).The 13C-depleted samples in P2 station could not have originated from vascularplant material because the low C/N ratios (Figure 5). Possibly they result fromgreater degradation of proteinaceous and carbohydrate components compared withlipid and lignin fractions (Benner et al. 1990). Since lipid and lignin fractions havemore negative �13C than proteins and carbohydrates, the selective loss of the latterfractions would result in lighter carbon isotope ratios (Cifuentes et al. 1996). Thesecond mechanism is 13C – depleted isotope ratios of POM resulted from phy-

Figure 7. Stable nitrogen isotope ratio of TSM vs Atomic carbon: nitrogen ratio (C/N).

42

toplankton assimilated 13C – depleted DIC, which is the product of active bacterialcontribution of the organic matter within the study area (Coffin and Cifuentes 1999).Future research that applies to survey carbon cycling in the shelf slope needs toaccount for factors that cause variation in the isotopic signature of the DIC pool.

Nitrogen isotope compositionsTwo-dimensional stable isotope analysis (�13C and �15N) provides a possibility tosurvey relative contribution of multiple sources of organic matter to an interestedregion. Figure 6 shows that the bottom of shelf water could be the fourth potentialsource of organic matter in the ECS shelf area except for the three distinct organicmaterial sources (the Changjiang contribution, in situ primary production in theshelf water and the Kuroshio intrusion). However, the relative contributions of theChangjiang Estuary and in situ primary production are dominant in most samples.The exchange of the shelf water and the Kuroshio Current is limited in autumnseason.

Although nitrogen stable isotopes have been successfully employed as organictracers in aquatic systems, dynamic cycling of nitrogen are subject to kinetic iso-tope fraction effects especially during the biogenic transformation and recycling ofdissolved and particulate nitrogen compounds (Gearing et al. 1984; Cifuentes et al.1988; Goering et al. 1990; Montoya 1994; Wu et al. 1999). It is expected that ni-trogen cycling in the bottom of shelf water would create isotopically enriched ni-trogen in the particulate organic matter through remineralization (such as �15N inE7 station). High �15N values correspond with high C/N ratios, which suggests thelatter are produced principally as a result of organic matter diagenesis (Figure 7)(DeMaster et al. 1985; Thornton and McManus 1994; Cifuentes et al. 1996). As(i.e., microbial mineralization) proceeds and the total amount of nitrogen presented

Figure 8. The estimated percentage of terrigenous POM distribution from the Changjiang to the shelfedge of ECS.

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is reduces, 14N is preferentially lost from the organic substrate, which become pro-gressively enriched in 15N. Consequently higher decomposed organic matter willcontain little nitrogen but with enriched in 15N. Such alteration of POM �15N hasbeen observed in other studies (Cifuentes et al. 1988; Goering et al. 1990; Montoya1994), and is regarded as one of the sources of organic nitrogen in this system.

However, in the Kuroshio waters it is not expected that higher �15N values cor-respond with higher C/N ratios (Figure 7). The source of this nitrogen contained inPOM was likely to be inorganic nitrogen that is assimilated into POM by microbialactivity (Coffin and Cifuentes 1999). Such conclusion is support with the charac-teristics of the Kuroshio water, which is rich in nitrate and ammonium resulted inremineralization of sinking particles (Liu et al. 1996). The average of �15N valuesof POM in the Kuroshio water is 5.6–7.8‰. This was similar to the average of�15N values of nitrate of the Kuroshio water (5.5 to 6.1‰), and suggests microbialassimilation of the inorganic nitrogen in to POM (Liu et al. 1996).

Cross shelf export of POCThe exchange of water masses between the ECS and the KW is a highlight in re-cent year (Nitani 1977; Tang et al. 1994; Yuan et al. 1999). Advective export ofPOC was observed in association with a cyclonic eddy at the shelf edge northeastof Taiwan (Liu et al. 2000). Water exchange in the PN section also showed a cy-clonic eddy: the KW inserts to the shelf area from the bottom and the subsurfacelayer and the SW extends to the Kuroshio area from surface and sub-surface. Thetotal amount of exchange what in autumn is estimated to be 1.17 × 106 m3·s−1,which is larger than the winter (0.90 × 106 m3·s−1) and less than the summer (3.22× 106 m3·s−1), similar to the spring (1.25 × 106 m3·s−1) (Lin et al. 1999). Theamount of KW intruding into the shelf is about 0.53 × 106 m3·s−1, while the exportof shelf water is 0.64 × 106 m3·s−1 (Lin et al. 1999). The inflow concentration istaken as the POC concentrations at 160 m in depth in the KW; the outflow con-centration is taken as the mean concentration in the top 50 m in the outer shelf.Thus the horizontal transport of POC across the shelf in the PN section can be de-rived from the mean POC concentration and exchange water volumes. The net ex-port flux of POC in the PN section is 4.1 kmol C/s and the annual export is 129Gmol C/yr. The POC export is more than the export of POC at the south ESC insummer (106 Gmol C/yr) but considerably smaller than the riverine inputs (750–875 Gmol C/yr, Cauwet and MacKenzie (1993)). The outflowing shelf water overthe whole ECS are estimated to export 506 ± 185 Gmol C/yr POC, which meansthe export of POC from the PN section could account to � 20% of the total (Chenand Wang 1999a).

Impact of changjiang over the shelf regionsTo better understand the carbon cycles and biogeochemical processes in the EastChina Sea, which is one of the most broad and productive shelf area in the world,it is important to study the contribution of terrigenous organic matter fromChangjiang over the shelf region. As mentioned before, the terrigenous inputs fromthe Changjiang, together with the contribution of KW and the marine POM by in

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situ primary production, the detritus from remineralization process, are the mainsources of POM in the ECS shelf. Then the contribution of individual sources/path-ways to the POM composition at marine station can be estimated by matrix arith-metic:

�a11 a12 ··· a1m

a21 ···

�

an1 an2 ··· anm��

X1

X2

�

Xm���

Y1

Y2

�

Yn� (2)

where (aij) represents the percentage concentration of the jth component in the ithsource material (Xi), Yi accounts for the ith element concentration in marine envi-ronment. The weight factor (aij in%) of various source contributions to the observedPOM composition can be solved by statistics (e.g., least-squares method) when m= n (Haswell 1992). In the case of m = n, a definite solution can be obtained. Inthis study we will use �13C,�15N and POC(%) of particulate matter to characterizeorganic matter sources. The four end members of particulates are listed in Table 1.

Figure 8 indicates the percentage of terrigenous POM distribution from theChangjiang estimated by this study, which shows the contribution of continentalmaterials upon the biogeochemical cycles of POM in the East China Sea. Within adistance of < 100 km off the river mouth, the terrigenous POM is dominated as70–80% of total POM where the biogeochemical cycling of POM is controlled bywater/sediment processes. The percentage of terrestrial POM decreases quickly atthe distance around 150 km off the mouth and only half of POM could be tracedfrom the continental inputs, where is also a productive fishing area and the fruitfulterrigenous POM are the important nourishing contributor. Further off shore (> 250km), the contribution of terrigenous POM is quite limited where the KW intrudesinto from the shelf edge and brings the organic matter from marine pool. Such dis-tribution is expected to be similar to the previous study of fluvial transport for par-ticulate heavy metals from the Changjiang into the East China Sea and the terres-trial POM can be traced over a distance of � 250 km over the shelf region (Zhang1999).

Table 1. The distinction of four end members in this study.

�13C �15N POC%

Terrestrial POMa −27 0.7 0.5

Kuroshio POMb −24 4 1.0

Marine POM −19 6.5 1.5

Detritus POM −22 9.5 0.06

aThe data of Terrestrial POM is referred from Wu et al. (2002); bThe data of Kuroshio POM is referredfrom Sheu et al. (1999) and Nakatsuka et al. (1997).

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Conclusion

The PN section is an ideal site to observe the active physical and biogeochemicalprocess that transport, transform, or exchange carbon. The vertical and geographi-cal variation of carbon and nitrogen isotope of POM showed predominance fea-tures that reflect the contribution of the terrigenous POM from the Changjiang andthe Kuroshio Water’s intrusion at the shelf edge. The incorporation of resuspendedsediment particles in the bottom layer was observed in this study too.

The distribution of terrestrial inputs was estimated by matrix arithmetic basedon the POM mass balance and the isotope values that only controlled by physicalmixing. It is expected that the continental material could disperse over a distanceof 250 km eastwards on the East China Sea shelf region.

The estimation of sources recorded by �13C data was partly confirmed by equiv-alent �15N and C/N compositions that reflected greater control by organic matterdiagenesis and biological processing. However, the lighter contribution of �13C dataof the Kuroshio samples also indicates the alteration of the isotope values by mi-crobial or other processes.

The net export flux of POC in the PN section is 4.1 kmol C/s and the annualexport is 129 Gmol C/yr and 23 Gmol C/yr more than the export of POC at thesouth ESC in summer, which could account to � 20% of the total (Chen and Wang1999a).

Acknowledgements

This study is funded by the Special Funds from National Key Basic Research Pro-gram of P. R. China (G1999043705) and of Natural Science Foundation of China(No. 40006008). The authors thank Drs. S.M. Liu, Dr J.L. Ren and Q. Wang fortheir assistance in field and laboratory work. The anonymous reviewers and DrBillen G. are thanked for constructive comments on the manuscript.

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